improve documentation regarding docker cluster
improve documentation regarding docker cluster

instead of trying to highlight stuff within markdown code blocks, this commits adds a descriptive sentence explaining the differences.

rnaveiras/etcd,CrazyJvm/etcd,mickep76/etcd,insequent/etcd,YuleiXiao/etcd,sebrandon1/etcd,lumjjb/etcd,etcd-io/etcd,akauppi/etcd,plasticbox/etcd,sinsharat/etcd,dannysauer/etcd,fanminshi/etcd,mkumatag/etcd,jlamillan/etcd,visheshnp/etcd,yudai/etcd,mqliang/etcd,cockroachdb/etcd,etcd-io/etcd,xiang90/etcd,plasticbox/etcd,ajityagaty/etcd,a-robinson/etcd,tguangch/etcd,bdarnell/etcd,ravigadde/etcd,rnaveiras/etcd,dannysauer/etcd,mkumatag/etcd,mickep76/etcd,bluebreezecf/etcd,yudai/etcd,etcd-io/etcd,visheshnp/etcd,li-ang/etcd,vimalk78/etcd,handsomesun/etcd,coreos/etcd,jlamillan/etcd,armstrongli/etcd,bdarnell/etcd,lorneli/etcd,sinsharat/etcd,handsomesun/etcd,coreos/etcd,rnaveiras/etcd,sebrandon1/etcd,ravigadde/etcd,vimalk78/etcd,whuwxl/etcd,heyitsanthony/etcd,joshix/etcd,a-robinson/etcd,bluebreezecf/etcd,xiang90/etcd,mqliang/etcd,li-ang/etcd,glevand/coreos--etcd,whuwxl/etcd,heyitsanthony/etcd,lorneli/etcd,joshix/etcd,armstrongli/etcd,ajityagaty/etcd,YuleiXiao/etcd,fanminshi/etcd,mqliang/etcd,insequent/etcd,CrazyJvm/etcd,lumjjb/etcd,mkumatag/etcd,akauppi/etcd,gyuho/etcd,glevand/coreos--etcd,tguangch/etcd,cockroachdb/etcd,gyuho/etcd,ajityagaty/etcd